Suspicion naturally falls on greenhouse gases and global warming as the cause, but there are a few other candidates for the honor. Whatever the reason, however, models that predict the future direction of climate change do not include the physics that drive many of the changes, so projections, such as the Intergovernmental Panel on Climate Change scenarios, are likely to be less severe than if these newer developments were incorporated.

"If we continue on with business as usual," said Richard Alley, professor geosciences at Pennsylvania State University, "we are likely to move CO2 (carbon dioxide) to levels that, when they last existed, there were no ice sheets on Earth."

Alley said there are many factors involved, and these elevated CO2 levels do not necessarily mean the loss of the ice sheets, which are very difficult to kill.

"It's much harder to get rid of an ice sheet than to create one," Alley said. "But I don't think we can exclude the possibility of losing one or both of the Greenland or west Antarctic ice sheets over centuries. Were it to happen -- and they are now changing 10 times faster than we're used to -- it moves Miami Beach north of the Everglades and maybe north of Florida."

The Antarctic ice sheet, for instance, contains most of Earth's fresh water and about 65 meters of sea level equivalent. In other words, if the Antarctic ice sheet melted -- currently considered an extremely unlikely scenario -- the sea level would rise about 213 feet.

The Greenland ice sheet contains about 7.5 meters of global sea level equivalent, or a little less than 25 feet of sea level rise.

About 100 million people currently live at 3 feet above sea level or less.

Every aspect of Earth's cryosphere is undergoing important changes, said Mark Serreze, associate professor of geography at the University of Colorado-Boulder. The cryosphere is the ice cover on Earth in various forms -- glaciers, sea ice, permafrost, snow cover and river and lake ice.

In the Arctic, Serreze said, there appears to have been a dramatic reduction in the extent of sea ice.

"The projection of changes in sea ice by three different climate models are that through 2050 we'll lose half the volume of Arctic sea ice," Serreze said.

This is based on projection of greenhouse gases and changes in the reflective ability of the Earth -- known as albedo. Ice and snow cover reflect sunlight back into space, tending to reduce the warming of the sun and the effects of climate change.

Many climate models up to now actually have predicted Antarctic glaciers would expand, for instance, a result now called into question by observations such as those cited by Alley.

Melting sea ice does not affect sea level. The ice already is floating in the ocean and has displaced the water. Like ice cubes melting in a vodka and tonic, it doesn't affect the level of the liquid.

If sea ice is reduced in extent, however, there is less reflection of sunlight and more absorption of heat, so there is a potential positive feedback effect where more warming might mean still more warming.

Changes in sea ice extent in the Arctic already have resulted in increased shore erosion. Entire villages have been moved as the soil on which they were built washes into the sea, Serreze said.

"There is a changing thermal energy state in the Arctic," Serreze said, "and it is tempting to attribute it to global warming."

Tempting but a shade premature, he added. The climate in the Arctic is strongly influenced by the North Atlantic Oscillation, a decadal variation in sea surface temperatures, winds and atmospheric pressures in the North Atlantic.

"From 1970 through the last five years, the NAO shifted into a positive phase, pumping a lot of heat and moisture into the Arctic," Serreze said. "This can explain a lot of the temperature trends in the Arctic -- warming, permafrost melting, reduced sea ice cover."

In addition, accompanying wind circulation also has changed, which affects the circulation of sea ice "pulling the ice off the coast of Siberia; thin ice, which can more easily melt," he said. So some affects may not be related to increased temperatures but rather to naturally cyclical atmospheric phenomena.

Still, there are some disturbing recent trends that do tend to implicate the warming climate.

"In the past five years," Serreze said, "the NAO has gone from a positive forcing to a more neutral state. Yet the Arctic is still warming, and the sea ice is still declining."

The past three years have shown a record minima in ice extent, even though the NAO is neutral.

"If we change the concentrations of greenhouse gases or stratospheric ozone toward a more positive state, will it bump the NAO toward this state? The jury is still out, but there is growing evidence that this is indeed the case," Serreze said.

There are several factors affecting sea level rise, the largest of which is the thermal expansion of the water. That is, heat expands so the warmer the water, the more space it takes up. Thermal expansion probably accounts for 3.1 millimeters per year of global sea level rise, according to Mark Meier, professor emeritus of geological sciences at CU-Boulder.

Much of the rest of the sea level rise to date -- an observed level of about 4.1 millimeters per year -- probably comes from melting glaciers.

"Glaciers are perhaps the most visible evidence of global warming," Meier said. "You have to believe this to believe your eyes."

At the current accelerated rate of ice loss, Meier said, "you get about 0.26 meters of sea level rise per year due to glaciers and ice caps alone," or a little less than a foot.

"The big ice sheet changes are interesting," Alley said. "The science is getting very exciting. There is a slow thickening inland and fast marginal thinning in some places. That's likely a signal of warming. The changes that are going on are not in the models that people used in the previous IPCC reports."

The scientists spoke at the annual meeting of the Geological Society of America being held this week in Denver.

--

A series by UPI examining the potential impact of global climate change. E-mail sciencemail@upi.com

United Press International is a leading provider of news, photos and information to millions of readers around the globe via UPI.com and its licensing services.

With a history of reliable reporting dating back to 1907, today's UPI is a credible source for the most important stories of the day, continually updated - a one-stop site for U.S. and world news, as well as entertainment, trends, science, health and stunning photography. UPI also provides insightful reports on key topics of geopolitical importance, including energy and security.